The invention relates to a jet spray method of applying adhesive to components of an electrochemical cell. In particular the invention relates to a jet spray method of applying adhesive to the inside surface of the cathode casing of a zinc/air cell.
There is a need to apply adhesive to electrochemical cell components, for example, portions of the inside surface of the casing for the cell. The portions of surfaces to be coated with adhesive can be very narrow or otherwise difficult to access using convention brushes or contact rollers. Many cells, such as conventional zinc/MnO2 alkaline cells include a plastic insulating plug which is inserted into an open end of the cell casing (housing) to seal the cell. There can be desirable benefits to applying adhesive sealant between the edge of such insulating plug and the cell casing, which is typically metallic. In such cells a metallic current collector in the form of an elongated nail is inserted through an aperture in the insulating plug so that the tip of the current collector passes into the anode mixture. It can be useful to apply adhesive sealant to the surface of the current collector or the insulating plug so that a tight seal develops when the current collector is inserted into the insulating plug. Conventional contact methods of applying the adhesive, for example, with brushes or rollers are usually slow or are difficult to apply to very narrow or difficult to reach surfaces.
Zinc/air depolarized cells are typically in the form of miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. There can be a problem of leakage of electrolyte from such cells if they are not properly sealed, particularly if the cell is misused. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each have a closed end an open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the cathode casing is typically inserted over the open end of the anode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising zinc, usually particulate zinc, with mercury optionally added to reduce gassing. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The closed end of the cathode casing (when the casing is held in vertical position with the closed end on top) can have a raised portion near its center or a flat bottom. This portion forms the positive terminal and typically contains a plurality of air holes therethrough. Cathode casings with a raised center on the closed end usually have an integrally formed annular recessed step, which extends from and surrounds the raised positive terminal.
The cathode casing contains an air diffuser (air filter) which lines the inside surface of the raised portion (positive terminal contact area) at the casing""s closed end. The air diffuser is placed adjacent to air holes in the raised portion of the casing closed end. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. The cathode material can be part of a cathode catalytic assembly which is inserted into the cathode casing so that it covers the air diffuser (filter). The cathode catalytic assembly can be formed by laminating a layer of electrolyte barrier material (hydrophobic air permeable film), preferably Teflon (tetrafluoroethylene), to one side of the catalytic material and an electrolyte permeable (ion permeable) separator material to the opposite side. The cathode catalytic assembly is then typically inserted into the cathode casing so that its central portion covers the air diffuser and a portion of the electrolyte barrier layer rests against the inside surface of the step.
In high drain or other demanding services, electrolyte can migrate to the edge of the catalytic cathode assembly and leakage of electrolyte from the cathode casing can occur. The leakage, if occurring, tends to occur along the peripheral edge of the cathode catalytic assembly and the cathode casing and then gradually seep from the cell through the air holes at the cathode casing closed end. The potential for leakage is also greater when the cathode casing is made very thin. For example, having a wall thickness of between about 4 and 10 mil (0.102 0.254 mm) or lower, for example, between about 2 and 6 mil (0.051 and 0.152 mm) in order to increase the amount of available internal volume. There is a greater tendency for the thin walled cathode casing to relax after crimping closes the cell. Such casing relaxation can result in the development or enlargement of microscopic pathways between the cathode catalytic assembly and the inside surface of cathode casing step, in turn providing a pathway for electrolyte leakage.
In commonly assigned U.S. Pat. No. 6,436,156 B1 a pad transfer method is disclosed for applying adhesive to the recessed annular step surrounding the raised terminal portion of the cathode casing of a zinc/air cell. The application of adhesive to the inside surface of the recessed step provides a tight seal between the cathode assembly and cathode casing of a zinc/air cell. The adhesive applied by pad transfer method prevents leakage of electrolyte around the edge of the cathode assembly and thus prevents electrolyte from escaping through air holes in the cathode casing.
An aspect of the invention is directed to a spray process for applying an adhesive sealant to components of an electrochemical cell. The adhesive is dispensed through a spray nozzle wherein the adhesive is applied in the form of a stream of droplets. In this regard the term xe2x80x9csprayxe2x80x9d or xe2x80x9cjet sprayxe2x80x9d as used herein shall be understood to mean the dispensing of a liquid through a nozzle so that it dispenses in the form of a stream of droplets. It has been determined that liquid adhesive of appropriate viscosity can be dispensed employing conventional micro-dispense technology, similar to that of ink jet spray technology. Such methods include dispensing the liquid adhesive employing micro-dispense nozzles in connection with thermal or piezoelectric ink jet spray methods.
An aspect of the invention is directed to a method for dispensing the liquid adhesive in the form of micro droplets. This can be accomplished by employing a piezoelectric nozzle. Such nozzle employs a piezoelectric transducer, which surrounds a resilient capillary nozzle formed of a resilient capillary tube which terminates in an outlet opening. The tube is preferably of glass. The piezoelectric transducer converts electrical pulses to mechanical vibrations, which in turn results in the harmonic vibration of the capillary nozzle. The rate of droplet propagation is responsive to and set by the frequency of the transducer. The frequency can be set so very high so that the distance between droplets formed are so small that the droplets tend to merge and the droplet propagation thus emulates a steady-stream. The droplet size can be adjusted by adjusting the size of the nozzle opening. Two distinct modes of operation can be employed: a) intermittent pulse and b) continuous pulse mode. For intermittent pulse dispensing, a set number of droplets are propagated over a set application cycle time. The desired rate of droplet propagation for intermittent pulse mode is between about 500 and 5000 droplets per second, more typically between about 1000 and 3000 droplets per second. The rate of droplet propagation is set by presetting the transducer frequency to a set value between about 500 and 5000 hertz, which corresponds to a droplet propagation rate respectively of between about 500 and 5000 droplets per second. The propagation of droplets at a preset rate is allowed to continue for a predetermined cycle time to give a desired number of droplets be cycle. Such application cycle can be repeated on an additional substrate or on the same substrate to provided layered adhesive application. The pause time between application cycles of droplet propagation can be set on the order of a second, hundredths of a second, and even thousandth of a seconds, or longer or shorter periods in order to meet desired throughput requirements. Thus, the pause time between application cycles is typically between about 0.001 and 1 seconds. During the application cycle when droplets are being propagated at a rate between 500 and 5000 droplets per second, which corresponds to the 500 to 5000 Hertz setting of the transducer, the application time can on the order of a second, hundredths of a second, and even thousandths of a second, or longer or shorter times. Thus, the application time is typically between about 0.001 and 1 second. An example of such dispensing would be for a setting of 3000 Hertz (3000 droplets per second) and the need to dispense 580 droplets to cover a linear distance of substrate of 0.933 inches (corresponding to approximate inside circumference of size 13 button cell), the dispense time (application cycle) would be 0.193 seconds. With droplet propagation at a frequency above about 5000 hertz the droplets tend to merge thereby emulating a continuous stream, with imperceptible spaces between droplets.
For continuous pulse dispensing, the droplet propagation rate is desirably between about 500 and 5000 droplets per second, more typically between about 1000 and 3000 droplets per second. In continuous pulse mode distinct droplets are continuously propagated until the signal to the transducer is shut-off by user intervention. Although such micro-dispense technology is normally employed in dispensing ink, such as with ink jet printers, it has been determined herein that liquid adhesive can also be dispensed using such method. The liquid adhesive desirably has a viscosity of between about 4 and 20 centipoise. The liquid adhesive can be dispensed from a micro sized nozzle, (a nozzle having an outlet opening diameter between about 50 and 60 micron) so that the width of the adhesive coating on the target surface may be very narrow, (between about 10 and 25 mil (0.254 and 0.635 mm)). The thickness of the adhesive (wet) transferred to the target surface may typically be between about 20 and 40 micron (0.020 and 0.040 millimeter) and even higher. The thickness of the adhesive (dry) may typically be about 10 micron (0.010 mm) and even higher.
A preferred adhesive is a solvent-based solution comprising polyamide adhesive resin. The adhesive component is desirably a low molecular weight thermoplastic polyamide resin. Preferred polyamide resins are available under the tradenames REAMID-100 and VERSAMID-100 (from Henkel Corp. or Cognis Corp.). These resins are gels at room temperature that are dimerized fatty acids with molecular weights around 390 and are the reaction products of dimerized fatty acids and diamines. Although higher molecular weight polyamide based adhesive components can be used, the lower weight components are preferred since they are more readily dissolved in the preferred solvent of choice. The adhesive component is dissolved in a solvent to the desired viscosity. Various solvents can be used, such as isopropanol or toluene, as well as mixtures of solvents. The preferred embodiment uses isopropanol as the solvent of choice due to its relative benign nature. An additional advantage of polyamides is their resistance to chemical attack by potassium hydroxide electrolyte.
The adhesive can be effectively applied to electrochemical cell components employing the jet spray method. The adhesive can be applied to provide an adhesive seal between desired surfaces of polymer components, between surfaces of metallic components or between surfaces of polymer and metallic components for the cell. For example, the adhesive can be applied to provide an adhesive seal between a plastic insulating plug and outer casing of an electrochemical cell to seal the open end of a metallic or plastic casing of a cylindrical or flat (prismatic) alkaline cell. In such cells there is typically an elongated current collector (nail) which is inserted through the insulating plug so that its tip penetrates into one of the electrode mixtures. For example, in zinc/MnO2 alkaline cells there is usually an elongated current collector nail inserted through an opening in the insulating plug so that it penetrates into the zinc anode mixture. Liquid adhesive can be applied around the surface of such current collector by the spray method of the invention so that an adhesive seal develops between the current collector and insulating plug. Alternatively, the sealant can be applied to the walls of the aperture in the insulating plug.
A particular aspect of the invention is directed a method of applying liquid adhesive by jet spray to a portion of the inside surface of a cathode casing for a zinc/air cell. The adhesive is applied, preferably by a piezoelectric nozzle, in the form of a stream of droplets. The stream of droplets can be applied to the inside surface of a recessed step, which extends from and surrounds a central positive terminal at the closed end of the cell""s cathode casing. The adhesive, upon drying, acts as a sealant to prevent leakage of electrolyte from the cell. If the closed end of the cathode casing is flat, that is, does not have a recessed step, the adhesive can be applied by a stream of droplets to the inside surface of the closed end along or near its peripheral edge.
In commonly assigned U.S. Pat. No. 6,436,156 B1 a pad transfer method is described for applying adhesive to portions of the inside surface of the cathode casing of a zinc/air button cell. Although the pad transfer method of applying adhesive is effective, the jet spray method of the present invention is believed to have additional advantages. Specifically, the jet spray method is faster than the pad transfer method and therefore results in higher output in number of units per hour of adhesive coated product. The jet spray method is a non contact method, that is does not involve contact of a pad, brush or applicator surface in order to apply the adhesive to the target substrate surface. This reduces the chance of applying too much adhesive, which could result in oozing from the edge of the target surface edge resulting in air-hole blockage or poor electrical contact between the cathode catalytic assembly and the cathode casing. Also, in the jet spray method the fine spray of liquid adhesive droplets can be aimed very precisely to the target surface. This makes possible precise application of a thin coating of adhesive to narrow width surfaces as in the recessed step of a cathode casing or to other difficult to reach surfaces of cell components. In addition, this method allows for highly accurate and repeatable metering of dispensed sealant weight, which in turn leads to more effective sealant-use management in production. The jet spray method is also suitable for applying adhesive in layers if desired, wherein each layer may be composed of the same or different material to produce an adhesive laminate.